[AMORO-4269] Refactor Iceberg thread pool initialization and scope

[xxubai]

Why are the changes needed?

Amoro uses deprecated Iceberg configuration and thread-pool APIs, while the planning and commit pool scope is not explicit enough for future process-specific scan/plan pools.

Close #4269.

Brief change log

• Replace deprecated SystemProperties and ThreadPools.newWorkerPool() usage.
• Initialize self-optimizing planning and commit pools from AMS configuration with explicit process-wide scope.
• Rename pool accessors and document their usage, leaving independent initialization available for future Iceberg process pools.

How was this patch tested?

• Add some test cases that check the changes thoroughly including negative and positive cases if possible

• Add screenshots for manual tests if appropriate

• Run test locally before making a pull request

  ./mvnw -pl amoro-ams -am -DskipTests compile

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable